Aromatic polyesters represented by polybutylene terephthalate (PBT) exhibit excellent heat resistance, chemical resistance and mechanical strength properties and are thereby used widely as engineering plastics. PBT itself has a rate of crystallization sufficiently high to permit it to be injection molded. However, further increases in the crystallization rate for PBT (thereby decreasing the molding cycle times, and increasing productivity) is desireable.
Copolyesters prepared by incorporating other comonomers into PBT or polybutylene naphthalate (PBN) generally exhibit remarkably low rates of crystallization because the comonomer acts as a crystallization inhibitor. However, such copolyesters typically exhibit poor moldability and mechanical strength properties. Furthermore, these copolyesters are problematic due to the propensity of the resin pellets to stick to one another during preparation. Other homopolyesters such as polyethylene terephthalate (PET), polypropylene terephthalate (PPT) and polyethylene naphthalate (PEN) have a low rate of crystallization in and of themselves, but are unsatisfactory in terms of their processability during molding, as well as their heat resistance and mechanical strength properties.
Many proposals have been made in an effort to enhance the crystallization rate of aromatic polyesters, including the addition of a crystallization accelerator. For example, the separate additions of (i) glass fibers (e.g. as disclosed in Japanese Patent Publication No. 18768/1970), (ii) an insoluble solid inorganic substance having a specified particle size (e.g. as disclosed in Japanese Patent Publication No. 9470/1970), and (iii) an ionic polyolefin copolymer (e.g., as disclosed in Japanese Patent Publication No. 26225/1970) have been suggested as crystallization accelerators for polyester resins. However, these crystallization accelerators cannot homogeneously be dispersed in polyester resins by melt-kneading techniques due to their poor solubility characteristics. Thus, the crystallization-accelerating effects of these conventional additives are still insufficient, particularly for copolyesters which are prepared using numerous comonomers. Adding increased amounts of such accelerators in an attempt to further accelerate the crystallization rate of a polyester resin is also impractical, because it significantly impairs the mechanical strength of the resin.
Another proposal to enhance the crystallization rate of polyester resins is to add an alkali metal salt of an aromatic sulfonic acid as disclosed in Japanese Patent Publication No. 56180/1985. Such an additive, however, is incompatible with the resin and thus does exhibit a sufficient crystallization-accelerating effect (even though it is superior to inactive powders in terms of dispersibility). It is further known that the high-melting polyester resin disclosed in Japanese Patent Laid-Open No. 149942/1983 has a relatively poor crystallization-accelerating effect, even though it is highly compatible with polyester resins. As described above, however, aromatic polyesters are typically not thought to exhibit satisfactory crystallization rates.
The present invention therefore, is broadly directed towards providing a polyester which exhibits excellent performance characteristics and towards processes for preparing the same. More specifically, the present invention is directed towards an aromatic polyester having a remarkably enhanced rate of crystallization as well as excellent mold-processability and mechanical strength properties which can be achieved by conducting transesterification in the presence of a specified monofunctional compound using a titanium compound as a catalyst, and then polycondensing the transesterification product.
The present invention, in preferred embodiments, relates to a process for preparing a polyester (and the polyester thereby prepared) by reacting a lower alkyl ester of an aromatic dicarboxylic acid with an aliphatic diol to form an aromatic polyester, whereby the transesterification of the lower alkyl ester with the diol is conducted in the presence of between 0.02 to 5.0 mole %, preferably between 0.02 to 1.80 mole %, and most preferably between 0.1 to 5.0 mole %, based on the lower alkyl ester of an aromatic dicarboxylic acid, of an aromatic sulfonic acid compound represented by the following general formula (I) using a titanium compound as a catalyst: EQU HO--R--O--Ar--SO.sub.3 M (I)
wherein --AR-- is a p-substituted benzene group or a 2,6-substituted naphthalene group; R is a divalent group selected from among --CH.sub.2 CH.sub.2, --CH(CH.sub.3)CH.sub.2 --, --CH.sub.2 CH(CH.sub.3)-- and CH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 --; and M is an alkali metal selected from among lithium, sodium and potassium.
The transesterified product is thereafter subjected to polycondensation conditions so as to obtain the aromatic polyester resin according to the present invention. When employed as a component in a blend (alloy) which includes a polyester base resin, the aromatic polyester resin according to the present invention serves as a useful crystallization rate accelerator when used, for example, in amounts between 1 to 100 parts by weight.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follows.